1. Field of the Invention
This invention relates to cathode electrode structures for sodium sulphur cells.
2. Prior Art
In a sodium sulphur cell, a solid electrolyte material, typically beta alumina, separates molten sodium forming the anode from a sulphur/sodium polysulphide cathodic reactant. On discharge of the cell, the sodium gives up electrons at the anodic interface of the solid electrolyte and sodium ions pass through the electrolyte into the cathode adjacent the opposite face of the electrolyte. The electrons pass through the sodium to the anode current collector and thence around an external circuit to a cathode current collector in the cathodic region of the cell. The electrons must pass from this cathode current collector to the region of the cathode adjacent the surface of the solid electrolyte; here they react with the sulphur to form sulphide ions. Sulphide ions and sodium ions form a polysulphide. The electronic conductivity of molten sulphur is low and hence it is the practice to pack the cathodic region with a fibrous carbon or graphite material to provide the required electronic conductivity, the fibrous material forming a matrix through which the cathodic reactant can move.
Sodium sulphur cells are commonly of tubular form and they may be of the kind known as a central sodium cell in which the sodium is inside the electrolyte tube and the cathodic region lies between the outer surface of the electrolyte tube and a tubular current collector, which might constitute or form part of the cell housing. Alternatively the cell may be of the type known as a central sulphur cell in which the sodium is outside the electrolyte tube and the cathodic reactant is in the annular region between the inner surface of the electrolyte tube and the central current collector rod or tube. In each of these constructions, the cathodic region is of annular form. It has been a common practice to use graphite felt as the electronically-conductive packing material in the cathodic region. Such felt may be formed for example into annular elements which may be packed axially into the cathodic region, the felt subsequently being impregnated with sulphur. It is also known for example from U.K. Pat. No. 1,472,975 and U.S. Pat. No. 4,052,535, to fabricate a cathode matrix from preformed elements which have been shaped by compression whilst impregnated with liquid sulphur, the elements being cooled whilst being pressed so that they can be fitted into the annular region of the cell. When the cell is raised to the operating temperature, the sulphur melts and the compressed fibrous material expands to provide good contact with both the current collector and the electrolyte surface.
In U.S. Pat. No. 4,076,902, there is described a sodium sulphur cell in which graphite fibres are arranged in an annular region between a beta alumina electrolyte tube and a surrounding cell housing with the fibres extending in a direction normal to the cathodic current collector constituted by the cell housing and thus normal to the electrolyte tube. It has long been appreciated that radial orientation of the fibres would be desirable to improve the radial conductivity but the above-mentioned U.S. patent specification does not disclose any method by which the graphite fibres can be so oriented. One method of radially orienting fibres is disclosed in U.S. Pat. No. 4,294,005. In this specification there is described a method in which a compressible block of electronically-conductive fibrous materials is formed with the fibres extending predominantly in parallel planes; the block is then cut in a plurality of parallel planes normal to a plane in which the fibres extend to form at least one sheet in which all the fibres have a component of direction normal to the plane of the sheet, the sheet is then compressed along a series of parallel regions which extend across the sheet in its plane to form segments of trapezoidal section between the compressed regions with the fibres having a component of direction normal to the parallel surface of the trapezoids. The fibrous sheet is impregnated either before compression or whilst compressed, with the cathodic reactant at a temperature such that the cathodic reactant is liquid and the compressed impregnated sheet is cooled to solidify the reactant. The segments of trapezoidal form thus form a plurality of segments which may be assembled together, each extending at least part of the lenght of the cell, to form an annular cathode electrode structure.